Perhaps what Mr. Coudet meant was that by providing a downsized Nikon jpeg, he would not be able to manipulate the file as he sees fit and he didn't have time to download the RAW.

What I meant was that by downsizing you threw away information. You asked to compare D800 and 5D Mark III, but by downsizing you threw away the advantage of D800 and influenced the results of such a comparison. Not a useful comparison.

If you'd actually looked at both photos at 21 MP, you would have seen that the D800 produces much finer details than the 5DmkIII.

The reason is simple: at full resolution, a bayer sensor has a lot of extrapolation going on. The real resolution of a bayer sensor is always lower than the effective pixels of it has (a 20MP sensor doesn't have 20MP rgb pixels, it has 5MP red + 10MP green and 5MP blue). Add to that the fact the there is blur filter in front of most cameras (anti-aliasing filter, the D800E doesn't have one), and it should be clear that a 20MP sensor doesn't resolve 20MP of details. There is an interesting experiment you can do: take any photo from a bayer sensor camera (almost all DSLRs) that has an anti-aliasing filter (for example the 5DmkIII), downsize it to 80%, and the upsize it back to the original resolution. You won't have lost any details.

Have you seen the comparison I posted above from both files at 16MP ? The D800E still has a clear edge, which means that the higher original resolution of the 36MP camera still shows.

Of course, Mr. Coudet chose to passive/aggresively denigrate

No. What I did was point out the problem, but chose not to elaborate on it because I (incorrectly) assumed that it should be obvious why you shouldn't downsize. That was my mistake. Your mistake was taking offense to my comment, where none was intended.